A semitransparent photovoltaic (PV) device, in a variety of form factors, can address a range of applications outside the scope of traditional PV modules. Examples include building-integrated photovoltaics, photovoltaic curtains, self-powered green-houses, and wearable electronics. Solution-processable organic and dye-sensitized solar cells (OPV and DSSC, respectively) were studied for this purpose over the past several years due to their potential for low-cost, roll-to-roll compatible manufacturing process, light weight, mechanical flexibility, and color tunability with different material set. However, the overall performance of these solar cells, in terms of their average visible transparency (AVT) and power conversion efficiency (PCE), needs further improvement to meet requirements for practical applications. Because the fabrication of semitransparent solar cells requires a trade-off between the PCE and transparency of devices, such high-performance devices are very demanding benchmarks in the field of solution processed photovoltaics.
In order to improve the figure of merit of solution processed semitransparent PV devices, the use of high efficiency, thin-film hybrid perovskite absorber materials is quite appealing. Recently, methylammonium lead trihalide (CH3NH3PbX3; X═Cl, Br, I) perovskites have rapidly emerged as a class of materials that may impact future PV technologies. The unique characteristics of these materials include excellent semiconducting properties, intense light absorption (absorption coefficient over 104 cm−1), tunable band gaps (1.2 to 2.3 eV), good crystallinity, low exciton binding energy, superior ambipolar carrier transport properties (charge carrier diffusion length over 1000 nm and lifetime over 100 ns), low temperature solution processibility and >15% PCEs (Certified 22.1%). Despite its relatively new entry in the PV field, only since 2009, its meteoric rise in PCE has made it an excellent contender in the field of thin-film photovoltaics. Efficient semitransparent solar cells would add further traits to the intriguing attributes of hybrid perovskites.
To fabricate high efficiency perovskite solar cells, typically, at least three different functional layers, an electron transport metal-oxide layer (e.g. TiO2), an intrinsic absorber (i.e. perovskite), and an organic hole-transport layer (HTL) (frequently spiro-OMeTAD) are stacked on top of each other, and the device is completed by forming proper electrode contacts (i.e. FTO or ITO on one side, Ag or Al on other side) on each side. However, the use of thick (≈200 nm) organic hole-transport layer leads to parasitic absorption losses, and thereby, reduces the overall transparency of the device stack. Also, the fabrication of this type of device layout involves high temperature (>400° C.) sintering step for metal-oxides, which limits the technology's true benefit of all low-temperature solution processibility, and preclude its applicability to flexible plastic based devices. Thus, perovskite device structures, devoid of metal-oxide interlayers, and comprised of the thinnest possible interlayers without significantly compromising the charge extraction, are highly desirable for low temperature, semitransparent solar cell fabrication.